Currently used pyroelectric imagers consist of a conventional vidicon, in which the photoconductive target is replaced by a thin, electroded pyroelectric disc. Radiation imaged on this disc is converted into heat by absorption. The resulting temperature pattern generates an equivalent charge pattern, which, in turn, is read out by an electron beam and displayed by a cathode ray tube.
The relatively high impedance of the electron beam and the capacitance of the disc determine the time constant of the signal discharge. To achieve efficient readout of the signal charge, this time constant must be shorter than the time the electron beam dwells on a picture element. This condition can be fulfilled if the relative dielectric constant is less than one hundred. Since the ratio of the pyroelectric coefficient to the dielectric constant is approximately constant for proper pyroelectrics, the large pyroelectric coefficient typical for materials of high dielectric constant cannot be exploited for pyroelectric vidicons.
The noise level of pyroelectric vidicon cameras comprises the shot noise of the electron beam, the pedestal current noise, and the preamplifier noise. Depending on design and frequency region, the total noise is proportional to the square root of the bandwidth of the video electronics or to a certain power thereof.
The prime objective of the invention is to overcome these basic limitations of the signal to noise ratio.